There is known a method for nondestructive inspection of atomic reactor pipelines, whereby pipeline portions to be inspected are connected to the arms of a bridge, whereupon alternating current is passed through the pipeline by an appropriate means electrically connected thereto; the degree of unbalance of the bridge is indicative of flaws in the pipeline portions being checked.
However, an unbalance of the bridge can only be observed in cases of major flaws. Minor flaws account only for negligible changes of the impedance which are commensurable with interfering factors, such as the temperature gradient between the pipeline portions connected to the arms of the bridge.
The method under review is disadvantageous in that it requires a great number of electric connections to the pipeline portions being inspected. Being a complication in itself, this also affects the accuracy of checking, especially in view of variable thermal and chemical conditions under which a pipeline normally operates.
The method does not make it possible to pinpoint the cause of an output signal. Signals of an equal magnitude may be caused by a single major defect or by a number of minor defects. There are cases when a reactor has to be shut down because of an inaccurate inspection.
True, the method does make it possible to establish the presence of a defect in a pipeline, but the exact location of the defect remain unknown.
There is further known a method for nondestructive inspection of pipelines, which consists in producing an alternating magnetic flux to interact with the wall of the pipeline, and measuring the parameters of the magnetic field brought about by eddy currents produced in the pipeline wall. The magnitude of the output signal of the measurement device is indicative of the state of the pipeline in the zone of interaction between the pipeline wall and the magnetic flux.
However, this latter method is not accurate enough. First, it is not sensitive enough to defects found deep inside the pipeline wall. Second, the method does not provide for a sufficiently high accuracy of measuring the parameters of the magnetic field produced by the eddy currents, keeping in mind that the magnetic field caused by the eddy currents has to be discriminated from the total magnetic field produced by both eddy currents and alternating magnetic flux.
There is further known a method for nondestructive inspection of pipelines (cf. U.S. Pat. No. 3,875,502 of 1975), which consists in using an alternating magnetic flux to produce an electromagnetic field and measuring the distribution of at least one parameter of the electromagnetic field in the zone of electromagnetic interaction between the magnetic flux and the pipeline. The distribution function of at least one electromagnetic field parameter, which is thus produced, is indicative of the state of the pipeline.
Like the previously discussed methods, this method lacks accuracy. This is due to the necessity of dscriminating the eddy current magnetic field from the total magnetic field, as well as to the strong non-uniformity of the total magnetic field. Besides, the method can hardly lend itself to the inspection of hard-to-get-at pipeline portions, especially at places where the pipeline is connected to other equipment, as well as at bends, at the locations of supporting structures, etc. The reason is the necessity of using exciting coils which envelop the pipeline in the inspection zone.
There is known a device for nondestructive inspection of pipelines, comprising means to supply alternating currents and voltages, means to connect them to the pipeline and thus produce a closed electric loop, and means for measuring electric characteristics of that loop for each frequency.
The measuring means is an impedance bridge whereof two arms are connected to portions of the pipeline that are to be inspected. The electric characteristic of the loop, which is to be measured, is the impedance of that loop.
However, the device under review does not make it possible to detect flaws at an early stage of their development, because such flaws can only be detected in the case of a significant change of the electric characteristics of the closed electric loop. Thus the device can only detect sufficiently serious damages.
Besides, the inspection of different sections of a pipeline requires a great number of electric connections between said connection means and the pipeline, which is not an easy task to perform, considering variable temperature and chemical conditions under which an atomic reactor pipeline normally operates.
The device cannot say if an output signal is caused by a single major defect or a plurality of minor defects. Thus a reactor shut-down and ensuing losses may happen without any real reason at all.
In order to pinpoint the location of a defect, the device in question must obviously be complemented with other devices, such as ultrasonic flaw detectors which necessitate a shut-down of the reactor to be put into operation.
Finally, it is impossible to inspect those points of the pipeline where it is connected to the measuring means.
There are further known devices for nondestructive inspection of pipelines of the type that comprises an a.c. source, a system of eddy current transducers of the kind that has to be put on the surface of an object to be investigated, a communication line, a unit for processing output signals of the eddy current transducers, and a data presentation unit.
The device is not accurate enough, which is due to its low sensitivity to defects found deep inside the pipeline walls and a low signal-to-noise ratio in cases the primary detectors are displaced in the inspection zone.
There is further known a device for nondestructive inspection of pipelines (cf. U.S. Pat. No. 3,875,502 of 1975), comprising an a.c. source connected to exciting inductance coils which contact the surface of a pipeline to be checked and thus produce a uniform magnetic field over a portion of the pipeline's surface. The device further incorporates a system of magnetic field measuring transducers arranged within said magnetic field, in immediate proximity to the pipeline's surface. It also includes a signal processing unit, a data presentation unit and a communication link to connect the system of magnetic field measuring transducers to the data processing unit, as well as to connect an inductance coil excitation means to said inductance coils. The a.c. source supplies alternating current which is passed through the inductance coils, whereby an alternating magnetic flux is brought about, which goes through the pipeline portion to be checked and produces eddy currents therein. The magnetic field measuring transducers are measuring inductance coils. These are arranged so that their axes are perpendicular to the magnetic lines, which rules out any electromagnetic interaction between the exciting coils and measuring inductance coils. A defect in the pipeline alters the eddy currents distribution and thus changes voltage across the measuring coils which interact with the eddy currents. The change of voltage is converted by the unit for processing the output signal of the magnetic field measuring transducers and recorded by the data presentation unit.
The latter device, too, is not accurate enough, which is due to a low signal-to-noise ratio because of possible displacements of the measuring transducers in the checking zone, as well as strong effects of electrically conducting objects, such as pipeline supports, on the results of measurements. The accuracy of inspection is also affected by different sensitivity of the measuring transducers to non-uniformities of the pipeline, and their low sensitivity to defects like transverse cracks which are most likely to occur in welds. Another reason for the low accuracy of checking the pipeline condition is the non-uniformity of eddy currents produced in the pipeline.
Finally, the device is such that the exciting coils have to be arranged right in the inspection zone, which makes the device too complicated and the inspection of hard-to-get-at places inconvenient.